FISSION OF METAL-CLUSTERS

Experimental results on the fission of doubly and other multiply charg ed metal clusters are reviewed and examined in the light of a simple m odel, where the fission barrier is approximated as two charged spheres in near-contact, at a mutual distance given by the balance between Co ulomb repulsion and attractive polarization effects. The barriers are estimated for different mass and charge splits and are compared with t he activation energy for the competing evaporation process. From the m odel as well as in experiment one finds a strong preference for singly charged trimers (with two electrons) in the ''fission'' channel, but also fragments with the higher magic electron numbers 8 and 20 may occ ur with enhanced abundance. In addition, there is a pronounced odd-eve n effect. In most of the experiments that have been carried out so far , fission occurs as the termination of a chain of evaporations of neut ral atoms. This limits the observations to a range where the surface e nergy dominates over the Coulomb energy of the fissioning cluster, exp laining the tendency for asymmetric fission and justifying the two-sph ere barrier approximation. Conditions favoring symmetric fission and o ther fission modes specific to highly charged metal drops are discusse d, and experimental approaches are suggested.